1. Field of the Invention
The present invention relates to a position sensitive proportional counter with delay line readout. It is related to the method to measure the spatial distribution of ionizing radiation emitted from a surface, --especially to measure the distribution of radioactive labelled thin layer chromatograms and electropherograms, --down to a range of the radiation to several tenth of a millimeter in air at normal atmospheric pressure.
2. Description of the Prior Art
The problem to be solved is to measure fast and in a simple way with high efficiency and high spatial resolution (1 mm and less) the spatial distribution of ionizing radiation, being emitted more or less isotropically, without the use of a mechanical collimator.
The classical method until now is the autoradiography. A very sensitive X-ray film is placed for some time above the surface to be investigated, e.g., a thin layer chromatogram plate. This method is very time consuming, e.g., up to several weeks, and no direct quantitative answer can be obtained. The quantitative measurement of the radioactivity can be made in scraping the radioactive areas (spots) from the thin layer plate and counting their activity by means of a liquid scintillation counter. In some cases the autoradiography is evaluated with a photo-densitometer. A much faster method, is the application of a spark chamber to detect the radioactive distribution, the sparks being recorded on film with a photo camera. Again, this method has similar draw-backs as the autoradiography for quantitative evaluation.
Since many years, thin layer radiochromatograms have been directly measured as they have been capable of not being destroyed with the so called thin layer scanner.
Such a thin layer scanner to measure the radioactive distribution of beta emitters from thin layer chromatogram plates is described in the German Patent DBP No. 1296 826. By means of a mechanical device, the thin layer plate is moved relative to the geiger-counter (or flow through counter) placed above the plate without touching it. The counter has a relatively small entrance window, about 1 mm.times.40 mm, to achieve the necessary spatial resolution in detecting radioactive labelled compounds, positioned very close to each other in the chromatogram.
With this device one scans in small steps (down to 1 mm) one surface element after the other of a thin layer plate. The measuring time is appreciably reduced compared to the autoradiography. Two disadvantages which cannot be overlooked, however, are that the spatial resolution is limited by the width of the entrance window and, therefore, also the efficiency. The measurement of a typical thin layer plate, 20 cm.times.20 cm, takes about 20 hours.
Much more sensitive would be, of course, a device capable of measuring quantitatively at once the radioactive distribution, not in successive steps, as the scanner does.
Such a device is known from 150, Journal of Chromatography, 409-418 (1978). The apparatus consists of a position sensitive proportional counter. The counting wire anode of high electrical resistance is on both ends connected to ground with low impedance. An ionizing particle, entering the counter generates at the wire, where it is passed, a current pulse, which at the two ends of the wire is divided in the same ratio as the resistance of the two wire parts, i.e., the two parts between the point of origin of the pulse where the particle has passed the wire and the two ends of the wire.
The two pulses at the end of the wire are amplified with charge sensitive preamplifiers, their pulse height being proportional to (1-x), where x is the distance of the primary charge pulse, i.e. where the particle has passed the wire, to one wire end.
This device has some disadvantages. The efficiency is rather low, about 0.5%. This is because only the very low energy beta particles are detected to obtain a good spatial resolution. For a typical beta emitter, as .sup.14 C, the spatial resolution is about 3 mm.
Another apparatus, developed by the Numelec Company to measure thin layer chromatograms is described in the cited Journal of Chromatography at page 411. The detector is a one dimensional gas flow through counter, which has incorporated parallel to the counting wire (anode), a cylindrical coil made of copper wire, which operates as delay line and gives the position information of the beta particles. The position of the beta particle at the wire is determined in measuring the propagation time of the induced signals into the delay line.
The internal volume of the counter has a height of more than 10 mm and the entrance window of about 250 mm.times.10 mm is closed off with a mechanical collimator. This collimator is made of thin metal walls (lamina), spaced about 1 mm from each other (like a venetian blind), arranged perpendicular to the wire, to eliminate particle tracks not travelling in a plane perpendicular to the wire. To achieve an efficient collimation, the thin metal walls (lamina) must have a minimum height, increasing therefore the distance between thin layer plate and counter wire, resulting in an important reduction of the detection efficiency of the detector.
The device has the following disadvantages:
1. The counter cannot be operated in the open mode. The entrance window has to be closed off with a thin foil or it has to make direct contact with the surface (thin layer plate) being investigated. In the last case a contamination from the radioactive sources being measured, cannot be avoided; also there is the danger of damage of the surface of the thin layer plate. Finally it is not possible to move automatically the thin layer plate relative to the counter, without direct mechanical contact. PA1 2. The incorporated collimator limits the spatial resolution, reduces the detection efficiency appreciably and deteriorates the ratio of signal to noise (background) due to scattered particles (radiation) at the collimator walls. PA1 3. The geometrical arrangement of the delay line relative to the counting wire is not optimal. The solid angle, wire-delay line, is only a small fraction of 2.pi.. Therefore the ratio signal/noise is not good, limiting the spatial resolution of the detector. The Numelec Company quotes a spatial resolution for .sup.14 C--radiation of 2 mm.